1. Field of the Invention
This invention relates to a method for controlling transportation of a print medium, such as paper and a film, to be printed in an inkjet printing apparatus, and relates to the printing apparatus using the method. More particularly, this invention is directed to a technique of controlling tension acting on the print medium transported by roll.
2. Description of the Related Art
The following has been conventionally known as an inkjet printing apparatus. That is, the inkjet printing apparatus feeds out a print medium wound in a roll form, such as web paper, and transports by roll the web paper. The inkjet printing apparatus then performs printing on the web paper with an inkjet print unit provided above a transportation path of the web paper. Thereafter, the inkjet printing apparatus dries ink adhering to the web paper with a drying section provided downstream of the printing unit. The inkjet printing apparatus finally winds the web paper in a roll form and collects the web paper.
In such the inkjet printing apparatus, uniform tension is applied to the web paper to be transported by roll. Examples of approaches for applying tension to the web paper to be transported by roll include draw control. In the draw control, each speed ratio between two adjacent drive rollers among two or more drive rollers is set in advance. The drive rollers are disposed along a transportation path for transporting the web paper. See, for example, Japanese Patent Publication No. 2000-262091. The draw control causes a difference in speed between the two adjacent drive rollers for transporting the web paper (i.e., a rotational speed of the drive roller downstream is greater than that of the drive roller upstream), resulting in application of tension to the web paper.
As the tension acting on the web paper to be transported varies, so-called shift register occurs. Accordingly, printing quality decreases. Thus a tension control method is adopted. In the method, tension of the web paper during transportation is detected, and speeds of drive rollers are operated such that the tension has a uniform value. See, for example, Japanese Patent Publication No. H07-196216.
However, the conventional examples with such constructions have the following drawback.
That is, when ink adheres to the web paper in the print unit, the web paper expands due to ink penetrated in the web paper. Such phenomenon may occur. Tension decreases as the web paper expands. Decrease of the tension is detected by a tension detecting sensor. Then, the rotation speeds of the drive rollers are controlled such that the detected tension has a given targeted value. Such a feedback control finally enables the tension to be maintained at a preferable value. On the other hand, a considerable time delay occurs from when the ink firstly adheres to the web paper to cause expansion of the web paper until when the feedback control leads to the tension maintained at a given value. As noted above, the tension acting on the web paper decreases during an initial printing period, resulting in decreased printing quality. Such a drawback may arise.